1. Field of the Invention
This invention generally relates to a developing device for developing a latent image, such as an electrostatic latent image, by application of a thin film of developer thereto for use in image processing machines, such as electrophotographic copiers, facsimile machines and printers. In particular, the present invention relates to a developer carrier for use in such a developing device for transporting the developer, typically toner, as carried thereon through a developing station where the latent image is developed and a method for manufacturing the same. More specifically, the present invention relates to a developer carrier suitable for use in a developing device employing magnetically attractable, electrically insulating toner as a developer and a method for manufacturing such a developer carrier.
2. Description of the Prior Art
In electrostatic recording machines, such as electrophotographic copiers, facsimiles and printers, the developing characteristics required for developing devices differ between the case in which an image to be developed mainly consists of a line image and the case in which an image to be developed mainly consists of an area image. The ideal developing characteristics are shown graphically in FIG. 1, in which the abscissa is taken for original image density and the ordinate is taken for copy image density. As shown in FIG. 1, the ideal developing characteristic required for developing an area image is indicated by a solid line A, while the ideal developing characteristic for a line image is indicated by a dotted line B. It may be seen that the rising slope is steeper for the case of line image (dotted line B) as compared with the case of area image (solid line A). The reason for this is that in the case of an area image, since sharpness of a developed image deteriorates if the original image density is lower, it is necessary to compensate for this by increasing the copy image density, whereas, in the case of an area image, sufficient sharpness may be obtained if the image density of a developed image is proportional to the image density of the original image.
It is common practice to utilize the so-called edge effect in order to attain an increased image density of a copy image for an original mainly consisting of a line image having a relatively lower image density. That is, with such an edge effect, the strength of electric field at the periphery of an electrostatic latent image is locally increased as compared with the strength of electric field at the central region of the latent image so that more toner may be deposited to the peripheral portion of the latent image. Thus, in the case where the latent image is a line image having a small or narrow area, the area of the latent image is substantially comprised of the peripheral portion which is subjected to the edge effect, thereby allowing to increase the image density of resultant developed image. The edge effect is sufficiently produced if use is made of the so-called two component developer containing toner and iron powder; however, the edge effect cannot be produced effectively in the case where use is made of a so-called single component developer comprised of magnetic toner and containing no iron powder.
Under the circumstances, there has been proposed a novel developing device including a developer carrier having a unique structure capable of producing the above-described ideal developing characteristics even if use is made a single component developer as disclosed in the Japanese Patent Application, No. 55-185726, assigned to the assignee of this application. The developer carrier disclosed in the above-noted patent application is schematically shown in FIG. 2 of this application and it comprises a cylindrical support 1 of electrically conductive material and an electrode layer 2 which is formed on the outer peripheral surface of the cylindrical support 1 from an electrically insulating material with a plurality of fine electrode particles 2a semispherical in shape being provided at the outer surface of the electrode layer 2 as uniformly dispersed axially as well as circumferentially, said individual electrode particles 2a being isolated from one another and maintained electrically floated. When the developer carrier shown in FIG. 2 is to be used as incorporated in a developing device employing a single component developer or magnetic toner, a magnet roller (not shown) is typically provided in an internal space 3 of the cylindrical support 1. With this arrangement, a magnetic field produced by the magnetic roller causes the magnetic toner to be attracted to the outer surface of the electrode layer 2.
FIGS. 3a and 3b show schematically how the developer carrier of FIG. 2 is effective in causing the edge effect to increase the image density of a line image when developed. In FIGS. 3a and 3b is shown a portion of a developer carrier 32, which structurally corresponds to the developer carrier shown in FIG. 2, as disposed opposite to a portion of a photosensitive member 31 on which a latent image (line image L.sub.1 in FIG. 3a and area image L.sub.2 in FIG. 3b) is defined by the positive charge. The photosensitive member 31 includes an electrically conductive substrate 31a and a photoconductive layer 31b formed thereon and like numerals are used for the elements of the developer carrier 32 to identify like elements of the developer carrier shown in FIG. 2. It is to be noted that, in fact, a layer of negatively charged magnetic toner should be present as formed on the surface of the electrode layer 2 of the developer carrier 32, this has been eliminated from these figures for the sake of simplicity. As indicated earlier, there are defined line and area latent images L.sub.1 and L.sub.2 at the outer surface of the photoconductive layer 31b, for example, from the positive charge, as shown in FIGS. 3a and 3b, respectively.
As may be easily understood, a layer of magnetic toner (not shown) carried on the developer carrier 32 is selectively transferred to the photosensitive member according to the charge pattern defined by the latent image L.sub.1, L.sub.2 so that the latent image L.sub.1, L.sub.2 is developed into a visible image. In this instance, the amount of toner deposition the latent image depends on the strength of an electric field present in the vicinity of the surface of photoconductive layer 31b so that the higher the strength of this electric field, the more the amount of deposition of toner to the latent image, thereby providing an increased image density in a developed image. Under the circumstances, in the case where the electrostatic latent image is a line image as shown in FIG. 3a, the strength of the electric field at the surface of the photosensitive member 31 where the line latent image L.sub.1 is formed is increased so that the amount of toner deposited to the latent image L.sub.1 becomes increased, thereby allowing to increase the image density of developed image, as compared with the case in which the electrode particles 2a are absent. The reason for this is that the provision of the electrode particles 2a causes the effective dielectric thickness between the line latent image L.sub.1 and its surrounding background portion to be thinner thereby increasing the number of electric force lines directed from the latent image L.sub.1 toward the surrounding background portion.
On the other hand, in the case where the electrostatic latent image is an area image as shown in FIG. 3b, the overall strength of electric field at the surface where the area latent image L.sub.2 is formed is not appreciably increased so that no significant changes in developing characteristic is produced due to the presence of the electrode particles 2a. In this case, the electric force lines directed from the latent image L.sub.2 to the conductive support 1 remain substantially unchanged with the presence of the electrode particles 2a excepting at the peripheral portion of the latent image L.sub.2 because the effective dielectric thickness is larger between the central portion of the latent image L.sub.2 and its surrounding background portion than between the latent image L.sub.2 and the conductive support 1. It should thus be apparent that the ideal developing characteristics shown in FIG. 1 may be obtained by using the developer carrier shown in FIG. 2.
However, difficulty has been encountered in manufacturing the developer carrier shown in FIG. 2, particularly in arranging the electrode particles 2a at the outer surface of the electrode layer 2. There has thus been necessity to develop novel structures and methods for manufacturing such structures with ease as well as at high accuracy.